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CIVIL RES-3ouRCES, LLC <br /> ENGINEERS & PLANNERS <br /> MEMORANDUM <br /> TO: Robert Lembke RECEIVED <br /> Red Tierra Project File—Section 20 Gravel Mine OCT 2 7 2022 <br /> FROM: Civil Resources, LLC <br /> DIVISION OF RECLAMATION <br /> DATE: October 14, 2022 MINING AND SAFETY <br /> RE: Modifications to Model in Response to DRMS Comments <br /> This memorandum summarizes the modifications to the Section 20 Groundwater Model Report dated August 2022 in response <br /> to a Division of Reclamation Mining and Safety (DRMS) comment that the Monarch DENM Gravel Mine, west of the site, be <br /> included in the Section 20 groundwater model (Adequacy Review dated September 12, 2022). Model modifications include: <br /> ➢ Added the Monarch DENM Gravel Mine slurry walls to the mitigation scenarios. <br /> ➢ An underdrain was included on the Monarch DENM Site. <br /> ➢ Due to the additional obstruction,the eastern mitigation system of an underdrain or dewatering wells pumping 2.5-3.25 <br /> cfs was modeled to be constructed at the same time as the Cell 4 slurry wall rather than during the construction of the <br /> Cell 5N slurry wall. <br /> ➢ Approximately 1,900 feet of underdrain was modeled to be constructed at the same time as the Cell 6 slurry wall to run <br /> between the Cell 6 slurry wall and Weld County Road 42. This drain was sized to remove 3.3 cfs from the model via <br /> discharge to the river which will require coordination with the Monarch DENM ownership group. <br /> The recommendations presented in the August 2022 report mitigated the groundwater mounding modeled to occur with full <br /> build-out of Section 20 slurry walls and the related rise in groundwater in the area of the Town of Gilcrest east of the Section 20 <br /> Site. The addition of the Monarch DENM resulted in further obstruction of groundwater flow around the west side of Section 20 <br /> resulting in additional regional mounding that requires a second underdrain system that conveys the flow to the river and limits <br /> the upstream groundwater rise via a westward draining underdrain beginning near the southwest comer of the Section 20 Site <br /> and carrying approximately 3.3 cfs to the South Platte River(approximately 1,900 feet long). <br /> Attachments: <br /> - DRMS Response Letter <br /> - Revised Groundwater Model Report <br /> PUnited Water&San-1411Section 20ORMSIGroundwatehMEM0 Groundwater Model Modifications docx <br /> B30B COLORADO BLVD, SUITE 200 0 FIRESTONE, COLORADO 8❑SO4 0 PHONE: 303.833. 1 41 6 <br />